Geoscience Reference
In-Depth Information
Vertical stress and pore pressure
Soil stresses are generated by weight (gravity) and loading. The vertical total
normal stress
v
in a horizontal layer increases with depth, according to (positive
z
is downwards)
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v
= z
(4.2)
Here,
is the volumetric (total) soil weight, composed of specific weight of
grains
s
and pore water weight
w
, incorporating saturation degree
s
, specific air
weight
a
and porosity
n
.
=
(1 -
n
)
s
sn
(1 -
s
)
n
a
(4.3)
w
In soil mechanics, the air or any other gas plays usually a minor role. The
groundwater or pore pressure
u
however plays an important role and in stagnant
groundwater it increases with depth, according to
u = z
w
(4.4)
Soil particles when submerged in groundwater loose some weight (Archimedes'
law or buoyancy force
15
), which affects their intergranular stress. In soil
mechanics, one adopts a hypothetical stress to express the average intergranular
stress, referred to by effective stress. For the vertical component it becomes with
(4.3), disregarding
a
v
' = z
'
with
' =
(1
- n
)
s
-
(
- sn
)
w
=
-
w
(4.5)
which for
s =
1 and
=
1 gives
' =
(1 -
n
)(
s
-
w
), exactly the submerged grain
unit weight.
For a saturated soil with relatively incompressible grains, fluctuations in a
(horizontal) free water level will not affect the effective vertical stress when fully
submerged. Thus, effective stresses in a sea bottom do not change due to sea
waves. However, fluctuations of a (horizontal) groundwater table will affect the
vertical effective stress because of the buoyancy. Large groundwater table lowering
may therefore cause noticeable land subsidence.
B
GROUNDWATER FLOW
Occurrence
Water penetrates the soil from rain, rivers, lakes, etc., accumulates in the pores
and evaporates through heat and plants. This is the field of geohydrology. One
14
Common in geotechnics, stress is defined positive for pressure, throughout this topic.
15
The pore pressure increases linearly according to volumetric water weight:
z
w
, but the
gravity force due to the mass of pore water increases only with
nz
w
. Archimedes' law
covers the difference
z
(1 -
n
)
w
.
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